The present invention relates to storage systems for storing data for a computer to read and write and more particularly, to a storage system based on a method for duplicating data possessed by the storage system.
Data of a disk subsystem (storage system) possessed by a main data center might be lost due to an accident such as an earthquake. In order to avoid such data loss, there is a method for recovering the data loss by previously creating a copy of the data in the disk subsystem (storage system), that is, by previously duplicating the data. The data duplicating method has been already put in practical use in the form of a so-called remote copying function.
The remote copying function is a function of not only providing a function of backuping the contents of data possessed by the main center into a remote center merely as data contents at a time point but also writing the data to be written even into a disk subsystem of the remote center when a host computer (host unit) of the main center issues a data write instruction to the disk subsystem. As a result, when some fault takes place in the system of the main center and this causes data of the disk subsystem of the main center not to be used, the processing of the main center can be immediately continued by using the data possessed by the disk subsystem of the remote center.
In this case, in order to continue the processing of the main center, it is necessary that the disk subsystem of the remote center have data consistent to the data of the main center at the time of the fault. In other words, the writing sequence of the data at the remote center must be consistent to the writing sequence of data at the main center.
Several techniques for holding such consistency of the data writing sequence are disclosed. For example, in U.S. Pat. No. 5,446,871 (JP-A-6-290125) and JP-A-11-85408, in a remote copying system for making a copy asynchronously with the data writing operation of a disk subsystem of a main center, a host unit or disk subsystem of a remote center executes write-data reflecting operation on the basis of time information attached to the data.
When a database is duplicated by a remote copy function, for example, data to be duplicated include, e.g., a data main body of the database and log data having data writing histories written therein. The data main body is closely associated with the log data, so that, a transaction of a data writing took place, the data writing transaction is completed not only by writing the data main body into the database but also by writing the write contents into the log data, thus ensuring the consistency of the data contents. In such a database that the consistency of the data contents is ensured by the aforementioned method, system design is often made so that the data main body and log data are recorded in different disk subsystem volumes from a viewpoint of reliability. Even when the data main body and log data stored in the respective volumes of the different disk subsystems are duplicated by the remote copying function, the contents of the data main body and log data copied to the volumes of the remote center must have a consistency. To this end, with respect to the remote copying function, even when the data main body and log data are written to the volumes of the disk subsystems in the main center, data writing must be carried out in the same data writing sequence as in the main center even in the remote center. In order to realize this currently, a pair of volumes is made by a volume (source volume of copy) possessed by the disk subsystem of the main center to be subjected to execution of a remote copy and a volume (target volume of copy) possessed by the disk subsystem of the remote center, a single group (which will be referred to as the volume group, hereinafter) is made by a set of such paired volumes and is collectively managed, whereby the data writing sequence at the remote center is held and the consistency of the data contents is ensured.
And when the data main body or log data is transmitted from the main center to the remote center, the transmission may sometimes be carried out via a public network.